1. Field of the Invention
The present invention relates to a configuration for performing accurate gray scale display in a display device having a light emitting element (a light emitting device), and a driving method thereof.
2. Description of the Related Art
In a conventional light emitting device, a pixel configuration as shown in FIG. 9 has been proposed in which a switching element 810 whose on/off is controlled by a video signal inputted from a signal line 814, a transistor 811 for driving a light emitting element 813, and a capacitor 812 provided between a power source line 815 and a gate electrode of the transistor 811 to hold a gate-source voltage of the transistor 811 (see Patent Document 1) are provided.    [Patent Document 1] Japanese Patent Laid-Open No. 2001-343933
It is considered that an equivalent circuit of a light emitting element described in Patent Document 1 can be shown by a parallel circuit including a diode 816 and a capacitor (CEL) in FIG. 9. Operation in the case where a current value of a current supplied to the light emitting element 813 varies is described below with reference to FIG. 9.
First, it is assumed that a current at a current value I0 flows constantly into the light emitting element 813. Then, in the case where the current value of the current flowing into the light emitting element 813 increases from I0 to I1, a current value of a current flowing into the diode 816 does not become I1 immediately. This is because an increased amount of the current value of the light emitting element 813 is equal to a sum of an increased amount of the current value of the current flowing into the diode 816 and a current value of a current flowing into the capacitor (CEL). Therefore, the current value of the current flowing into the diode 816 becomes equal to I1 when the charging of the capacitor (CEL) is completed.
Meanwhile, assuming that the current at the current value I0 flows constantly into the light emitting element 813 and then the current value decreases from I0 to I2, a sum of the current value of the current flowing into the diode 816 and a current value of a current discharged from the capacitor (CEL) becomes I2. The current value of the current flowing into the diode 816 becomes equal to I2 when the discharging of the capacitor (CEL) is completed. In the above-described cases, the time until which the current value of the constant current flowing into the diode 816 changes is equal to the time until which changing of a potential between an anode and a cathode of the light emitting element 813 is completed, which becomes longer as the size of the capacitor (CEL) is larger and as the changed amount of the current value of the light emitting element 813 is larger.
The pixel circuit shown in FIG. 9 further includes overlap capacitance (Cgd) between a gate electrode and a drain electrode of the driving transistor 811 and parasitic capacitance (Cp) caused by overlap between the gate electrode and the anode and the like depending on the layout in addition to the capacitor (CEL) between both the electrodes of the light emitting element 813.
At this time, the switching element 810 is turned on, and a current corresponding to a gray scale signal inputted into the gate of the transistor 811 is supplied to the light emitting element 813 and an anode potential thereof changes. However, when the capacitor (CEL) of the light emitting element 813 is large and a changed amount of the current value of the current supplied to the light emitting element 813 is large, it takes a long time to complete the charging/discharging of the capacitor (CEL) and complete the changing of the anode potential. Therefore, there is a case where the changing of the anode potential does not complete in the on-period of the switching element 810.
Then, in the case where the anode potential of the light emitting element 813 changes (a value of change is ΔVA) after the switching element 810 is turned off in FIG. 9, the potential of the gate electrode of the transistor 811 changes due to capacitive coupling of the parasitic capacitance (Cp), the overlap capacitance (Cgd), and the capacitor (CS) 812. A value of change at this time, ΔVB is expressed by ΔVB=(CP+Cgd)/(CP+Cgd+CS)×ΔVA.
As set forth above, in the case where the potential of the gate electrode of the transistor 811 changes after a gray scale signal is inputted into each pixel, there is a problem in that the current value of the current supplied to the light emitting element 813 changes so that accurate gray scale display cannot be obtained. In particular, in the case of performing black display, a current may flow into the light emitting element so that clear black display cannot be easily performed.